A passive optical network (PON) is a telecommunications network that uses point-to-multipoint fiber to the premises in which unpowered optical splitters are used to enable a single optical fiber to serve multiple premises. A PON consists of an optical line terminal (OLT) at the service provider's central office and a number of optical network units (ONUs) near end users. A PON reduces the amount of fibers and central office equipments required compared with point-to-point architectures. A passive optical network is a form of fiber-optic access network.
At the April 2012 meeting of the Full Service Access Network (FSAN) group, time and wavelength division multiplexed passive optical network (TWDM-PON) technology was chosen as the primary solution for next-generation passive optical network stage-2 (NG-PON2) architecture. Such passive optical network PON are notably used in local access networks (LAN).
In time and wavelength division multiplexed PON, tunable transmitters are required at the Optical Network Unit (ONU). It is therefore desired to design a cost-effective wavelength tunable laser source for time and wavelength division multiplexed PON.
For this, it is known to use an external cavity laser or sample grating distributed Bragg reflector lasers.
However, as these kinds of laser sources are developed mainly for long-haul applications, the resulting cost is too high for applications in local access networks. In addition, a temperature control is required to assure a single mode operation of the laser source and to allow the control of the wavelength emitted by the laser source.
DFB lasers are also used for time and wavelength division multiplexed PON. A distributed feedback laser (DFB) is a type of laser diode, quantum cascade laser or optical fibre laser where the active region of the device is periodically structured such that a diffraction grating is incorporated. The structure builds a one dimensional interference grating (Bragg scattering) and the grating provides optical feedback for the laser.
Nevertheless, the tuning range of such type of laser source is limited since the wavelength tunability is of the order of 3.2 nm (corresponding to a thermal tunability of roughly 0.08 nm/K and a temperature change of 40 K). In addition, the output power of the laser source is reduced along the wavelength tuning due to the heating carried out for obtaining wavelength tuning.